URL Encode options passed to Spreedly#subscribe_url

Hey there. I noticed while looking around that passing the email test+email@example.com to Spreedly#subscribe_url would result in the Spreedly page showing: "test email@example.com".

Included is a quick patch to URL encode the options along with updated tests.

I struggled to get the testing environment set up so I could only really run the non-mechanize tests. In case you care I have a branch where I added a basic Gemfile / .rbenv_version file to get me moving: https://github.com/dies-el/spreedly-gem/tree/bundler_rbenv

Thanks,

Andrew